Series crystal phase modulation receiver



' M. e. CROSBY 2,156,376

Filed Dec. 8, 1937 2 Sheets-Sheet 1 ATTORNEY.

INVENTOR. MURRA V G. CROSBY SERIES CRYSTAL PHASE MODULATION RECEIVER May2, 1939.

May 2, 1939. M. G. CROSBY SERIES CRYSTAL PHAS E MODULATION RECEIVERFiled Dec. 8, 1937 2 Sheets-Sheet 2 Fig. 2a

. 5&3 mutt FREQUENC Y INVENTOR. MU AV 6. CROSBY Fq'9.2d

U I L A TTORNEYQ Patented May 2, 1939 UNITED STATES SERIES CRYSTAL PHASEMODULATION RECEIVER Delaware Application December 8, 1937, Serial No.178,655

8 Claims.

This application concerns a phase modulation receiver of the crystalfilter converting type, in which two separate crystals are used toobtain the required overand under-neutralized filter characteristicswith demodulating means of the back-to-back type coupled with the outputof the filters. The characteristic of the filters comprising theunderand over-neutralized crystals is such that when phase modulatedwave energy is passed by the filters, it is converted to waves ofamplitude modulation and the filter outputs are so arranged and coupledto the backto-back demodulators that the modulation envelops resultingfrom the conversion have a 180 relation so that the detector systemcoupled to the filters detects the amplitude modulations, resulting fromthe conversion, to render the signal. Undesired amplitude modulationsreaching the filters have similar effects on the output thereof and are,due to the back-to-back effect in the d'emodulators, cancelled orsubstantially cancelled by diiferential action.

In my United States application Serial #167,- 344, filed October 5,1937, I have described a receiver of the general type involved here. Thepresent receiver, however, includes various improvements and novelfeatures not included in the said prior application. One differencebetween the receiver of the prior application and the receiver of thepresent application resides in the manner of connecting the crystalfilters. In the prior application, energy is fed from an impedance orreactance to the crystal through a resistor and the output of the filtercomprises the drop across the crystal. In the receiver of the presentapplication, the crystals in their holders are connected in seriescircuits so that at the resonant frequency of the crystal a maximumamount of wave energy is passed thereby, While at the anti-resonantfrequency of the p-iezo-electrio crystal a minimum amount of energy ispassed. The anti-resonant frequency characteristic of the crystal is duetothe holder capacity and the inductive effect of the crystal per se.With the crystals connected in series as in this application, a moreeflicient transfer of energy from the input reactance or impedance tothe filter outputs is obtained due to the fact that there is no loss inthe resistor which has been used heretofore for feeding the crystals inthe shunt connection. I have also found that the series connection asused in the present invention gives a greater exaltation of the carrierwith respect to the side bands. This is of material advantage inreducing the distortion due to fading of the character duringtransmission.

In describing my invention, reference will be made to the attacheddrawings wherein;

Figure 1 illustrates the essential elements of phase modulated Waveenergy receiving, amplifying and converting means with automaticfrequency control means, all arranged in accordance with the presentinvention. Certain of the elements of the receiver are showndiagrammatically by rectangles since these elements per se form no partof the present invention. The filter circuits for converting phasemodulations on the wave energy to characteristic amplitude modulationsare illustrated in detail, although in the sake of simplicity, eventhese circuits do not include all known refinements which applicantmight use in a practical application.

Figures 2a to Zn inclusive, are resonant curves, reactance curves, andwave component vector diagrams used in illustrating the operation of theconverting circuit.

Referring to the drawings, Figure 1 shows a somewhat complete receiveremploying the principles involved here. In this receiver, wave energy tobe demodulated is intercepted by an aerial system A and supplied by A toa radio frequency amplifier 5 wherein the energy is selected andamplified as desired. Energy from the output of 4 is supplied to a firstdetector 6 also coupled to a local oscillator O of the controllabletype. Oscillations from 0 beat with the wave energy in 6 to produceintermediate frequency energy supplied to intermediate frequencyamplifier 8. The output of 8 supplies potentials to the primary windingof a transformer l 0, the secondary winding of which has one terminalconnected to ground and the other terminal connected to electrodes orholders of piezo-electric crystals 24 and 25. The primary and secondarywindings of [0 are tuned to substantially the mean frequency of theintermediate frequency energy by condensers l2 and i6, respectively. Inorder to pass substantially uniformly all of the frequencies involved,the primary and secondary windings are shunted by damping resistors 9and I8, respectively. An electrode of crystal 2% is connected to thecontrol grid 35 of a coupling and amplifying tube 34, while an electrodeof piezo-electric crystal 25 is coupled to the control grid 37 of acoupling and amplifying tube 36. The crystal 25 has its two holders orelectrodes shunted by an inductive reactance 26, the purpose of whichwill appear in detail hereinafter. Grid leak resistors 30 and 30' areconnected between the control grids 35 and 31, respectively, and groundG, while selfbiasing resistors shunted by b-y-passing condensers 40 andM are connected between the cathodes of the tubes 3% and 36,respectively, and ground.

The anode-electrode of 36 is connected with the primary winding of acoupling bandpass transformer 56, while the anode of 36 is coupled tothe primary winding of a bandpass transformer 52. The bandpasstransformer 56 may be tuned to the desired frequency by tuning condenser55 and may be damped by damping resistor 5| to give it the desiredbandpass characteristic. In like manner and for the same purpose, theprimary winding of transformer 52 may be tuned by condenser 56 andclamped by resistor 53. The secondary windings of 52 and 56 areconnected in series between the anodes of diode rectifier 69 and 6d. Thecathodes of diode rectifiers 66 and 64 are connected together byresistors 63 and 65, the resistors being shunted by by-passingcondensers as shown. The cathode end of the resistor 65 is connected toground so that potentials at the cathode end of 63 vary with respect toground in accordance With the combined outputs of 60 and 64, which inturn are excited by the energy passed by 50 and 52. The latter energyis, due to causes which will be described hereinafter, amplitudemodulated energy resulting from the conversion of the phase modulationsin the crystal filters. Moreover, the amplitude modulations have opposedenvelopes. The resulting amplitude modulations are impressed bycondenser 69 on the input electrode 12 of an audio frequency amplifier18, the anode of which is coupled to a transformer M having a secondarywhich may supply the amplified modulation potentials to any utilizationcircuit. Potentials from the resistors 63 and 65 are also supplied byway of a time constant control circuit 6! to the control grid 82 of amodulator tube which controls the frequency of the oscillator Osupplying oscillations to the first detector for beating purposes.

The modulator M comprises a reactance tube having an anode 84 coupled tothe anode 93 of the oscillator 94. The anode to cathode impedance ofmodulator tube 80 is in shunt to the reactance 96 in the oscillationgenerating circuit 94 so that it controls the frequency of theoscillation generated. The anode 84 of tube 88 also is connected by wayof a blocking condenser 9! to its grid 86 through phase shiftingresistor 99 and condenser 88. Resistor 69, which is properly by-passedby a condenser, furnishes self-bias for tube 89. The grid 86 of tube 89is coupled to the connections between 90 and 88. Oscillator tube 94 hasits grid 98, anode 93, and cathode I89 coupled in a frequencydetermining and stabilizing circuit 96 which circuit is coupled by leadsto the first detector 6. This reactance circuit and oscillator are asdescribed in my copending application Serial #167,344 filed October 5,1937.

The value of resistance 98 is high as compared to the reactance ofcondenser 88 for the frequency used, that is, generated in 0, so thatthe current through this circuit is largely resistive and is in phasewith the voltage. However, the voltage drop across 88 leads the currentby 90 and the phase quadrature relation between the radio frequencypotentials on 84 and 86 necessary for the reactive effect is obtained.The reactance of tube 88 shunts the frequency determining circuit 96 andconsequently the reactance tube controls to some extent the frequency ofthe oscillations produced in 96 and tube 94. This reactive effect, whichmay be considered inductive or capacitive, is, in turn, controlled bythe potential supplied from the time control circuit 6| to the grid 82of reactance tube 89. The potential supplied to 82 is a function of themean frequency of the intermediate frequency energy supplied by 56 andchanging when the intermediate frequency energy drifts in eitherdirection from the selected mean frequency. Such drift may be caused bythe change in frequency of the received wave or the frequency of theoscillator O, or both, and results in a change in potential developed in63 and 65, a change of the potential of 82 and a correcting change inthe reactance reflected by the reactance tube 80 into the circuit 96 ora part thereof. Since the plate 84 is connected to the plate 93 and thegrid 86 is excited by voltage displaced in phase relation to that of thecircuit 96, the plate current in 86 is likewise out of phase with thevoltage of and the control tube 86 looks like a reactance to the circuit96.

The operation of my receiver and in particular of the convertingcircuits including the crystals 2% and 25, will be more clearlyunderstood by reference to Figures 2a to 2h, inclusive, of the drawings.Crystals 24 and 25 are ground to the same frequency, but crystal 25 hasan inductance 26 in shunt with the electrodes so that this crystalholder will have an inductive instead of a capacitive effect. The holderreactance of crystal 24 is left capacitive. Since one of the crystals isshunted by capacity only and the other is shunted by inductance, therequired overand underneutralized characteristics will be obtained dueto the fact that an inductance in shunt with the crystal is equivalentto an over-neutralization. Thus, the capacitive crystal holder, e. g.,24, will effect an under-neutralized characteristic and the inductiveholder, e. g., 25, will effect an overneutralized characteristic.

The reactance characteristic of crystal 24, which is capacitive, is asshown in Figure 2b. The voltage from transformer ll! is fed directly tothis circuit and the drop across the crystal is inductive or capacitive,depending on the frequency of the wave supplied by H), and, inaccordance with the curve of Figure 21). It can be seen that in theregion where the side bands are disposed, the crystal is capacitive onboth sides of the carrier frequency except for a small interval near thecarrier frequency. Due to this fact, the phase of the side bands isshifted 90 with respect to the carrier as shown in Figures 2a and 201.Figure 20 represents the phase modulated wave supplied from I!) to beconverted, while Figure 2d represents the wave to which the phasemodulated wave has been converted and has the characteristics of anamplitude modulated Wave. Note that the arrows denoting rotation of theside bands in Figure 201 show that they rotate in a direction such as toapproach cancellation of the carrier which corresponds to a minimumpoint on the amplitude envelope of the resultant wave. The amplitudecharacteristic of this filter, including crystal 24, has been shown atFigure 2a.

Crystal 26, which is shunted by the inductance 26, has a reactancefrequency characteristic as shown in Figure 2). In this figure, thefilter is inductive on both sides of the carrier except for a smallportion of the frequencies adjacent the carrier. Due to this reactiveeffect, the phase of the side bands is 90 lagging instead of leading asWas the case with crystal 24. Consequently,

the phase modulated wave of Figure 2g is converted to the amplitudemodulated wave of Figure 2h, Note that in the amplitude modulated waveof Figure 2h, the side bands are approaching the condition of aiding thecarrier which corresponds to a maximum point on the amplitude envelopeof the resultant wave.

The amplitude characteristics of the filter 25 are as shown in Figure26. From an examination of Figures 2d and 2h, due to the manner in whichthe side bands combine with the carrier, it can be seen that the phasesof the amplitude modulation envelopes resulting from the conversion bythe two filters is out of phase. Consequently, the detection of thisamplitude modulation and combination by the series connection of thediodes 60 and 64 and diode resistors 63 and B5 combines the phasemodulation outputs additively. The resulting potentials of modulationfrequency are amplified in 10 and utilized in 14, while the resultingpotentials due to slow changes or drifts in the mean frequency actthrough 6| to correct the oscillator frequency.

Amplitude modulation, both noise and signal, which is present on thewave being received by this receiver either is not detected or isbalanced out before appearing at the receiver output terminals. The sidebands of amplitude modulation, after being shifted in phase by thefilters, are converted to phase modulation which is not detectable onthe amplitude detectors. In the frequency range close to the carrierwhere the side bands are not shifted, or for the amplitude modulationwhich is present due to the fact that the filters may not completelyconvert the amplitude modulation into phase modulation, the pushpullconnection of the diode detectors is such as to cause the amplitudemodulation output from one detector to cancel that from the other.Evenharmonic detector distortion appearing in the output of eachdetector due to the detection of the desired modulation, is alsocancelled or compensated in the output.

I claim:

1. In a system for converting phase modulations on wave energy tocorresponding amplitude modulations, a filter circuit which iscapacitive on both sides of the frequency to which it is resonant as toa frequency spectrum of the order of a modulation frequency band, asecond filter circuit which is inductive on both sides of the frequencyto which the filter circuit is resonant as to a frequency spectrum ofthe order of a modulation frequency band, an impedance on which phasemodulated wave energy may be impressed, pairs of output terminals, andmeans connecting said impedance and each of said filters in a seriescircuit including a different pair of said output terminals.

2. In a filter system for converting phase modulations on wave energy tocharacteristic amplitude modulations, pairs of output terminals, acrystal in a holder in an under-neutralized filter circuit resonantsubstantially at the mean frequency of the phase modulated wave energyto be converted, a second crystal in a second holder shunted by aninductance to form an over-neutralized filter circuit substantiallyresonant at the mean frequency of the wave energy to be converted, meansfor impressing phase modulated Wave energy on said filter circuits, andmeans for connecting each of said filter circuits in a series circuitwith a different pair of said output terminals.

' pairs of terminals, a second piezo-electric crystal connecting a pointon said reactance to the remaining terminal of the other of said pairsof terminals, and an inductive reactance in shunt to one of saidpiezo-electric crystals.

4. In a system for converting phase modulations on wave energy tocorresponding amplitude modulations, a reactance, means for impressingphase modulated wave energy on said reactance, a pair of electrondischarge devices each having input electrodes and having outputelectrodes coupled to a utilization circuit, a connection between aninput electrode in each of said devices and a point on said reactance,separate piezoelectric crystals connecting another point on saidreactance to an input electrode of each of said devices, and a reactancein shunt to one of said crystals.

5. In a system for converting phase modulated wave energy intocorresponding amplitude modsaid reactance to the remaining terminal ofoneof said pairs of terminals, a second piezo-electric crystalconnecting another point on said reactance to the remaining terminal ofthe other of said pairs of terminals, an inductive reactance in shunt toone of said piezo-electric crystals, and a rectifier system having inputelectrodes coupled in push-pull relation to said pairs of terminals andoutput electrodes connected. to a utilization circuit.

6. In a system for converting phase modulations and Wave energy tocorresponding amplitude modulations and demodulating the same, areactance, means for impressing phase modulated wave energy on saidreactance, a pair of electron discharge devices each having inputelectrodes and having output electrodes, a connection between the inputelectrode in each of said tubes and a point on said reactance, separatepiezo-electric crystals connecting other points on said reactance to aninput electrode of each of said tubes, a reactance in shunt to one ofsaid crystals, and a detector system having input electrodes coupled inpush-pull relation to the output electrodes of said devices and outputelectrodes coupled to a utilization circuit.

7. In a system for converting phase modulated wave energy tocharacteristic amplitude modulated wave energies with envelopes. ofopposed phase to substantially eliminate or cancel undesired amplitudemodulations on said wave energy, a reactance excited by said waveenergy, a rectifier system having input electrodes and a common outputcircuit, a pair of piezo-electric crystals in holders, a pair ofelectron discharge coupling tubes each having input and outputelectrodes, means connecting the output electrodes of said tube to theinput electrodes. of said rectifier system, a circuit including one ofsaid crystals connecting the input electrodes of said tube in serieswith said reactance, a circuit including the other of said crystalsconnecting the input electrodes of the other of said tubes in serieswith 10 crystal in a second holder in a filter circuit, said secondcrystal having dimensions such that it is resonant substantially at themean frequency of the Wave energy to be converted, means foroverneutralizing one of said crystals, means for impressing phasemodulated wave energy on said filter circuit and means for connectingeach of said filter circuits to a pair of said output termjnals.

MURRAY G. CROSBY.

